WASHINGTON — Gun-rights advocates in the Senate will try to turn a bill expanding background checks for firearms buyers into a vehicle to boost Americans’ ability to carry concealed weapons.

The bill, what remains of President Barack Obama’s gun- safety agenda after efforts to ban assault rifles and high- capacity ammunition magazines were dropped, could be so turned upside down that supporters might choose to back away from it altogether.

Republican Sen. Marco Rubio of Florida said people like him who have concealed-carry permits should be allowed to take their weapons to other states, even those that don’t permit concealed guns. Adding that proposal, which came within two votes of Senate adoption in 2009, would make it tougher for gun- control advocates to support the legislation.

“That would be a bitter pill for gun-control advocates,” said Dennis Henigan, former vice president of the Washington- based Brady Center to Prevent Gun Violence, which advocates stricter gun laws. Such a law would “block states like California and New York from enforcing their strict laws against out-of-state visitors who have concealed-weapon permits,” he said in a telephone interview yesterday.

The debate over gun control was reignited by the Dec. 14 shootings at Sandy Hook Elementary School in Newtown, Conn. Adam Lanza used a Bushmaster AR-15 semiautomatic rifle to kill 20 children and six school employees. Obama’s gun- safety plan included an assault-weapons ban and limits on ammunition magazines, though they were removed from the Senate bill amid opposition from the National Rifle Association.

The current Senate measure would expand background checks of gun purchasers, increase funding for school safety and set new penalties for gun trafficking. It gained momentum last week when West Virginia Democrat Joe Manchin and Pennsylvania Republican Pat Toomey agreed on a bipartisan background-check plan, which will be the first amendment to receive a vote.

Senate Majority Leader Harry Reid has promised votes on additional amendments to S. 649, including the assault-weapons and ammunition proposals.

The proposal to require all states to honor concealed-carry permits issued by any state has been “a major priority of the gun lobby,” Henigan said. “In terms of the list of poison pills, that may be right at the top” as an amendment that could scuttle the gun legislation.

Sen. Charles Schumer, D-N.Y., last week called the concealed-carry measure one of the most “pernicious” amendments Republicans could seek.

About half of the 50 states have some type of conceal-carry law.

Among those who have sponsored the concealed-carry proposal is Sen. Mark Begich, D-Alaska, one of two Democrats who voted last week to block the gun legislation from advancing. He and Sen. Mark Pryor, D-Ark., who cast the other Democratic vote opposing debate on April 11, are seeking re-election next year in states won in November by Republican presidential candidate Mitt Romney.

Rubio, during an April 14 appearance on ABC’s “This Week,” said laws like Florida’s conceal-carry permit statute should be made applicable nationwide to people who qualify for the gun license.

State conceal-carry permits should also be honored at gun shows in place of a background check, Rubio said, because “someone who has a conceal-carry permit has been background- checked.”

Henigan countered that “a lot of states that have very liberal concealed-weapons laws have virtually no standards at all for issuing permits.”

Expanding the use of conceal-carry permits has had widespread support in the Senate, where the proposal won 58 votes in 2009 when South Dakota Republican John Thune offered it as an amendment to a defense bill. Reid, a Nevada Democrat, was among those voting for the proposal, which fell two votes short of the 60 votes needed for adoption.

Rubio and other Republicans say there aren’t enough prosecutions of people who fail criminal background checks when trying to purchase weapons. Sen. Lindsey Graham, R-S.C., said April 10 that an amendment to expand prosecutions is expected to be introduced during the Senate debate.

Graham said he probably will offer his proposal to expand the national criminal-information database to include hundreds of thousands of people legally declared mentally incompetent.

Manchin and Toomey’s proposal would expand the current law requiring background checks for gun purchases from federally licensed dealers to include people who buy from private dealers at gun shows or over the Internet. It would exempt non- commercial gun sales or transfers between family members.

“If you are a law-abiding gun owner you are going to like this bill,” Manchin said Monday during floor debate. He and Toomey are promoting their compromise as protecting Second Amendment rights to gun ownership.

Mandatory background checks for most gun purchasers are supported by 91 percent of U.S. voters, including 96 percent of Democrats, 88 percent of Republicans and 88 percent of gun- owning households, according to a Quinnipiac University poll conducted March 27-April 1.

If the bill passes the Senate, it faces an uncertain fate in the House, where there is widespread Republican opposition. Speaker John Boehner of Ohio said he won’t make a “blanket” commitment to bring a gun measure to the House floor.